1. Field of the Invention
The present invention relates to a surface acoustic wave apparatus for use in, for example, a resonator, band-pass filter, or other suitable device. In particular, the present invention relates to a surface acoustic wave apparatus using a rotated Y plate X-propagating LiTaO3 substrate and a method for manufacturing the same.
2. Description of the Related Art
In mobile communication apparatuses, for example, cellular phones, surface acoustic wave filters have been used as RF stage band-pass filters and duplexers. As this sort of surface acoustic wave filter, a surface acoustic wave filter using a leaky surface acoustic wave has been used and includes an IDT (Interdigital Transducer) made of Al arranged on a 30xc2x0 to 50xc2x0-rotated Y plate X-propagating LiTaO3 substrate.
However, this surface acoustic wave filter has a poor frequency temperature characteristic of xe2x88x9230 ppm/xc2x0 C. to xe2x88x9240 ppm/xc2x0 C. and, therefore, improvements thereto are required. In order to improve the frequency temperature characteristic, a structure has been suggested, in which an IDT made of Al has been formed on the 30xc2x0 to 50xc2x0-rotated Y plate X-propagating LiTaO3 substrate and, thereafter, a SiO2 film has been further laminated. The frequency temperature characteristic is improved by the arrangement of the SiO2 film.
When the IDT made of Al is formed, the electrode film thickness H/xcex (H denotes a film thickness and xcex denotes a wavelength of a surface acoustic wave) of the IDT is significantly increased to 0.08 to 0.10 in order to increase the reflection coefficient and electromechanical coefficient K2. Since the IDT made of Al was allowed to have a significant thickness as described above, regarding the portion shown in FIG. 18A, when the SiO2 film was formed thereon in order to improve the frequency temperature characteristic, large height differences occurred in the SiO2 film and, thereby, cracks sometimes occurred in the SiO2 film as shown in FIG. 18B. Consequently, the filter characteristics of the surface acoustic wave filter were degraded due to the occurrence of the cracks.
In addition, since the electrode film thickness of the IDT made of Al is large, an effect of covering the differences in the electrode surface of the IDT based on the arrangement of the SiO2 film was not adequate and, thereby, the temperature characteristic was not always improved adequately.
Furthermore, the attenuation constant was increased due to the arrangement of the SiO2 film and, thereby, degradation of the filter characteristics occurred.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave apparatus and a method for manufacturing the same, wherein the surface acoustic wave apparatus includes a rotated Y plate X-propagating LiTaO3 substrate having a unique arrangement of a SiO2 film such that the frequency temperature characteristic is greatly improved, the electrode film thickness of the IDT can be significantly reduced, cracking in the SiO2 film is prevented and, furthermore, the attenuation constant is reduced by a large degree, so that targeted electrical characteristics, for example, desired filter characteristics, can be attained, and the electromechanical coefficient and reflection coefficient in the IDT are allowed to have adequate values.
According to a preferred embodiment of the present invention, a surface acoustic wave apparatus includes a LiTaO3 substrate having Euler angles (0xc2x13xc2x0, 113xc2x0 to 136xc2x0, 0xc2x13xc2x0), at least one IDT which is arranged on the LiTaO3 substrate and which primarily includes Cu, and a SiO2 film arranged on the LiTaO3 substrate to cover the at least one IDT.
In preferred embodiments of the present invention, since the IDT primarily includes Cu, and the SiO2 film is arranged in a unique manner, the electromechanical coefficient is increased, and the frequency temperature characteristic is improved. Furthermore, since the LiTaO3 substrate having the specific desired Euler angles is used, the attenuation constant xcex1 is greatly reduced.
In another preferred embodiment of the present invention, preferably, the film thickness H/xcex of the IDT normalized by the wavelength of the surface acoustic wave is preferably within the range of about 0.01 to about 0.08, and the film thickness H/xcex of the SiO2 film normalized by the wavelength of the surface acoustic wave is preferably within the range of about 0.15 to about 0.40. In that case, according to preferred embodiments of the present invention, a surface acoustic wave apparatus which has a large electromechanical coefficient and reflection coefficient, excellent frequency temperature characteristic, and adequately reduced attenuation constant and in which cracking in the SiO2 film are prevented from occurring can be provided with reliability.
In a preferred embodiment of the present invention, preferably, the film thickness H/xcex of the IDT is about 0.12 or less, and the combination of the normalized film thickness of the SiO2 and the Euler angles of the LiTaO3 substrate is any one of the values shown in the following Table.
In another preferred embodiment of the present invention, preferably, when the normalized film thickness of the SiO2 film is represented by hs, the xcex8 of the Euler angles (0xc2x0, xcex8, 0xc2x0) fall within the range of the following Formula (1):
xcex8minxe2x88x922xc2x0 less than xcex8xe2x89xa6xcex8min+2xc2x0xe2x80x83xe2x80x83Formula (1)
where in the Formula (1), when the normalized film thickness H/xcex of the IDT is within the range of the following (a) to (e), the value of xcex8min is represented by the following Formulae A to E, respectively.
(a) when 0 less than H/xcexxe2x89xa60.01
xcex8min=xe2x88x92139.713xc3x97hs3+43.07132xc3x97hs2xe2x88x9220.568011xc3x97hs+125.8314xe2x80x83xe2x80x83Formula A
(b) when 0.01 less than H/xcexxe2x89xa60.03
xcex8min=xe2x88x92139.660xc3x97hs3+46.02985xc3x97hs2xe2x88x9221.141500xc3x97hs+127.4181xe2x80x83xe2x80x83Formula B
(c) when 0.03 less than H/xcexxe2x89xa60.05
xcex8min=xe2x88x92139.607xc3x97hs3+48.98838xc3x97hs2xe2x88x9221.714900xc3x97hs+129.0048xe2x80x83xe2x80x83Formula C
(d) when 0.05 less than H/xcexxe2x89xa60.07
xcex8min=xe2x88x92112.068xc3x97hs3+39.60355xc3x97hs2xe2x88x9221.186000xc3x97hs+129.9397xe2x80x83xe2x80x83Formula D
(e) when 0.07 less than H/xcexxe2x89xa60.09
xcex8min=xe2x88x92126.954xc3x97hs3+67.40488xc3x97hs2xe2x88x9229.432000xc3x97hs+131.5686xe2x80x83xe2x80x83Formula E
In a preferred embodiment of the present invention, preferably, the combination of the normalized film thickness of the SiO2 and the Euler angles of the LiTaO3 substrate is any of the values specified in the following Table.
In the case of the combinations shown in the aforementioned Table, that is, in the case where the xcex8 of the Euler angles is specified to be about 125xc2x0 or less, the electromechanical coefficient K2 is be further increased.
In the surface acoustic wave apparatus according to a preferred embodiment of the present invention, preferably, a leaky surface acoustic wave including a shear horizontal wave as a main component is used as the surface acoustic wave. According to preferred embodiments of the present invention, a surface acoustic wave apparatus having an excellent frequency temperature characteristic, an IDT with a large electromechanical coefficient and a reflection coefficient, and a small propagation loss can be provided.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.